Somali pirates hijack another boat – or two or three – every day despite the best efforts of the U.S. 5th Fleet, NATO, Russia, India and others. This is not new; pirates have been frustrating the mighty for at least 2,000 years – since snatching Julius Caesar, then ransoming him for 50 talents, and leading Alexander the Great on a wild goose chase around the Mediterranean. As the U.N. Security Council grapples with Somali piracy, its members are in venerable company.

According to Plutarch, Caesar got even: He caught and crucified his hapless captors, but he never drove piracy from the Roman Empire. Today's brigands have faster boats and rocket-propelled grenades, but their best assets are the same as those used to stymie Caesar and Alexander – big seas, many prey and few protectors. For Somali pirates today, the odds are even better: They have a hunting ground of 2.5 million square nautical miles (about five times the size of the Gulf of Mexico) transited by 20,000 commercial ships annually. Muscle-bound warships try but regularly fail to defend all those merchantmen.

It may be time for a new strategy. This week, the U.N. Security Council gave Secretary General Ban Ki-moon 90 days to come up with one. The usual nostrums do not look promising. Late last month, for example, the council adopted a British plan for travel and financial sanctions against the pirates' leading lights. That's fine, but the pirates seem to have their own means of travel and finance, including million-dollar bounties that fall from the sky in suitcases – and no one seems to know who the leaders are, anyway. The Organization of African Unity wants a U.N. peacekeeping force to tame Somalia, but the United Nations has sought recruits for months without success. Private security guards will shoot it out with bandits for $5,000 to $20,000 a day, but many ships carry flammable cargos, so seaborne firefights between thugs and testosterone-soaked mercenaries are best avoided.

The global shipping association wants to seal off Somalia with a blockade. Since the country has a 2,050-mile coastline, the longest in Africa, that's a little ambitious. But the shippers are on the right track. Somali pirates need havens that have water deep enough not to run trophy ships aground yet are close enough to towns for resupply. From the capital, Mogadishu, north around the Horn of Africa into the Gulf of Aden, Somalia has only a few suitable places: Eyl, Hobyo and Haradhere on the Indian Ocean; Bosaso on the Gulf of Aden; Mogadishu itself; and possibly one or two more. Separate the pirates from those havens, and their cost-risk ratios may once again favor fishing.

At each of those ports, cooperating naval vessels could establish a sort of police line to challenge, board and inspect suspicious craft, both leaving and returning. Evidence of piratical acts or intent would trigger confiscation of their boats, weapons and materials and the detention of crews for prosecution. One small naval ship per port, equipped with a helicopter and smaller boats, would suffice. The dozen ships patrolling these waters could handle this task with ships to spare for pirate duty farther out at sea.

A legal basis for a pirate blockade exists. In fact, piracy has been a crime under international law since the 17th century. Building on customary law and the U.N. Law of the Sea Convention, the Security Council in June authorized a six-month mandate, which this week was extended for a year, for cooperating states to use force against pirates in Somali territorial waters and on the high seas. Because piracy is a crime of universal jurisdiction, captors with no connection to the victims, property or perpetrators may detain and prosecute suspects. For those who prefer to extradite suspects from Somalia, nearby Kenya is an option; its courts have been convicting pirates for years.

Alongside those sturdy criminal and jurisdictional foundations is the Security Council's own authority to impose blockades, under Article 42 of the U.N. Charter, to counter threats of any stripe to international security. Because the multinational flotilla already in place and the European Union force en route to Somalia essentially operate independently, a clear mandate would facilitate agreement on a number of key issues – tasks, tactics, communications, rules of engagement and logistics. A limited blockade would also cost less than the combined insurance premiums, security charges, expenses from longer voyages and million-dollar bounties likely to be paid if piracy continues. Shippers could cushion the blow to national treasuries by paying into a multinational fund – set up outside the United Nations – to support the operation.

The concept of a blockade was rejected last month by a NATO spokesman as something not “contemplated” by the Security Council. Now, though, it should be. Trillions of dollars in commercial cargos transit the sea lanes annually; so long as they do, thugs in boats will prey on them. The world has a chance to shut down the Somali pirate franchise. Let's not squander it.

Fromuth, a lawyer, served at the State Department and the U.S. Mission to the United Nations during the Clinton administration.